applied fracture mechanics

236Applied Fracture MechanicsProcesses of nucleation and formation of fatigue slips bands often precedes at grainboundaries as is shown in figure 2. Mughrabi et al. [29] showed that nucleation at grainboundaries occur at sites where persistent slip bands (PSBs) impinge.In smooth surfaces of SS’s, during the early corrosion fatigue damage, the permanent slipbands are associated with breakdown of the passive film. The breakdown of the passive filmcauses dissolution of the non-covered active metal in contact with the electrolyte, which actsas a small anode against the large cathodic. This small anode is a vulnerable area on themetal surface, where a high local metal dissolution takes place, and leading cavities such aspits and/or crevice. Extensive development of techniques to study electrochemical reactionshave simulated the analysis of potential and current fluctuations taking place duringelectrochemical processes which are translated in to electrochemical noise (EN) signals.Figure 2. Evolution of fatigue slip bands in a 316L SS during cycling stress in air, under max=217 MPa(90%0.2), load frequency ω=1.0 Hz and a stress ratio R=0 conditions.EN is defined as spontaneous fluctuations in current and potential generated by corrosionreactions [30-34]. In this context, the study of spontaneous current or potential fluctuationsto characterize corrosion processes have received considerable attention, such as the studyof corrosion potential (Ecorr) and current fluctuations applied to monitor the onset of eventscharacterizing pitting [35-36] or stress corrosion cracking (SCC) [37-39] The noise is typicallymeasured potentiostatically, galvanostatically, or in a Zero Resistance Ammeter (ZRA)mode [30,31,34]. With ZRA mode, both the potential and current fluctuations can bemeasured using two nominally identical electrodes, connected through a ZRA; where a netcurrent and changes in the potential are observed at free corrosion conditions according tothe corrosion process.During the Corrosion Fatigue tests conducted in natural seawater [16], the potential timeseries obtained from electrochemical noise measurements had a common characteristicpattern of quick drop and slow recovery. Common explanations for these patterns of quickdrop and slow recovery have been associated to anodic dissolution, passive filmbreakdown, metastable and stable pitting corrosion, or with the crack initiation and crackgrowth due to microstrain and local mechanical stress conditions [16].The figure 3 shows patterns of quick drop and slow recovery, indicating that passive filmeventually is broken, due to applied mechanical stress and the interaction with theelectrolyte, turning it in an active dissolution of metal. Also, on figure 3, the surface damageafter N=43200 cycles of corrosion fatigue tests can be observed. In 316L SS-natural seawater